Image sensors are devices which receive an optical signal from an object and convert the optical signal to an electrical signal. The electrical signal can then be transmitted for further processing, such as digitization and then storage in a storage device such as a memory or optical or magnetic disk, or for presentation on a display, printing, etc. Image sensors are typically used in devices such as digital cameras, camcorders, printers, facsimile machines, etc.
Image sensors are typically of two types, namely, charge coupled device (CCD) sensors and CMOS image sensors (CIS). CCD sensors typically have advantages including low noise operation and device uniformity. CIS devices are typically characterized by low power consumption and can be operated at high speed due to a high frame rate capability.
FIG. 1 contains a schematic partial cross-sectional view of an image sensor in accordance with the prior art. The device of FIG. 1 includes a semiconductor substrate 1 on which are formed isolation regions 3, which define pixel active regions 3a therebetween. Multiple photosensing elements 8, which include n-type photodiodes 5 and p-type hole accumulated device (HAD) regions 7, are formed on the substrate 1 in the pixel active regions 3a. A first interlayer dielectric layer 9 is formed over the photosensing elements 8. A plurality of first interconnection lines 11 are formed over the first interlayer dielectric layer 9. A second interlayer dielectric layer 13 is formed over the first dielectric layer 9 and the first interconnection lines 11. A plurality of second interconnection lines 15 are formed on the second interlayer dielectric layer 13. A passivation layer 19 is formed on the second interconnection lines 15. Color filters 23 are formed as shown on the passivation layer 19 over the active pixel regions 3a, and a flattening layer 25 is formed over the color filters 23. Microlenses 27 are formed on the flattening layer 25 over the color filters 23.
The passivation layer 19 is used to prevent internal circuits from external environmental effects such as moisture. Silicon nitride (SiN) is usually used as a passivation layer. However, the SiN has a higher photon absorption rate and a higher refractive index than other commonly used dielectric films such as silicon dioxide (SiO2). As a result, the amount of incident light 29 that reaches the photosensing elements 8 for detection by the sensor is reduced by absorption and refraction as the light passes through the passivation layer 19. As a result, the sensitivity of the sensor is reduced.